1. Field of the Invention
The present invention relates to a method and system for removing dust and SO.sub.x from exhaust gas in a coal-fired boiler.
2. Description of the Related Art
FIGS. 4 and 5 are block diagrams of conventional systems for removing dust and SO.sub.x from exhaust gas in a coal-fired boiler.
With reference first to FIG. 4, the system includes a coal-fired boiler 1. The temperature of exhaust gas from the boiler 1 is reduced to 120.degree. to 160.degree. C. in an air preheater 2. Dust is removed from the exhaust gas in a dry electrostatic precipitator 4 until its concentration is reduced to about 100 mg/m.sup.3 or slightly higher. Furthermore, in order to save the energy necessary to reheat the gas after desulfurization, heat is recovered in a rotary-reheat type of gas-gas heater 7. Thereafter, the temperature of the exhaust gas is reduced to its saturation temperature in a cooling/dust removing section 6a of a wet type of exhaust gas desulfurization unit 6, and the dust is further removed from the exhaust gas. The concentration of SO.sub.x is reduced in a SO.sub.x absorbing section 6b provided separately from the cooling/dust removing section 6a. Finally, the exhaust gas is reheated in the gas-gas heater 7 and is then discharged through a chimney.
This prior system suffers from the following problems.
(1) As shown in FIG. 6, the temperature of exhaust gas in the electrostatic precipitator is high, and the specific resistance of dust produced by the burning of some types of coal is above 10.sup.11 .OMEGA.-cm. When the specific resistance of the dust exceeds 10.sup.11 .OMEGA.-cm, reverse ionization occurs in the electrostatic precipitator. This substantially deteriorates the performance of the electrostatic precipitator. For this reason, a large electrostatic precipitator is needed to collect dust at a required rate.
(2) If the concentration of dust at the outlet of the electrostatic precipitator is reduced to 100 mg/m.sup.3 N or lower, then SO.sub.3 remaining in the gas is atomized when the exhaust gas is cooled by the gas-gas heater. SO.sub.3 thus atomized is then deposited in the gas-gas heater. This results in the corrosion of members made of the same metal such as carbon steel, stainless steel, etc. It thus becomes necessary to raise the concentration of dust above 100 mg/m.sup.3 N so as to neutralize SO.sub.3 as a countermeasure against the corrosion problem. As a result, the concentration of dust is approximately 20 mg/m.sup.3 N at the outlet of the desulfurization unit 6. If dust leakage (approximately 10%) takes place in the gas-gas heater 7, then the concentration of dust at the inlet of the chimney is reduced only to 30 mg/m.sup.3 N at most.
(3) The desulfurization unit uses a lime (limestone, hereinafter)-gypsum method. When it is attempted to recover the by-product gypsum for reuse, if the concentration of dust at the inlet is 100 mg/m.sup.3 N or more, the dust deteriorates the purity of the gypsum. In order to prevent such a problem and maintain the purity of gypsum at a predetermined level, the desulfurization unit must sometimes be of separation type including a cooling/dust removing section 6a and a separate absorbing section 6b, particularly in the case where the concentration of SO.sub.2 at the inlet is low. This results in an increase in the consumption of space and in the running cost.
FIG. 5 shows a system used when the emission standards for dust to be discharged from the chimney are more severe, for instance, when the concentration of dust at the inlet of the chimney must be reduced to 10 mg/m.sup.3 N or less. This system includes a leak-free type of gas-gas heater wherein heat exchange is effected through a heating medium. The equipment in this system is made from economical carbon steel. This system is different from the system of FIG. 4 in that a heat recovery section 3a is separated from a reheater section 3b, and in that a wet electrostatic precipitator 8 is provided downstream of the purifier 6. However, this system suffers from the problems (1) and (3). In order to reduce the concentration of dust from 20 mg/m.sup.3 N to 10 mg/m.sup.3 N, a wet electrostatic precipitator is used which takes up more space and increases the running cost of the system.